Engineering genetically encoded fluorescent indicators for imaging of neuronal activity: Progress and prospects

•Overview of recent advances in engineering genetically encoded fluorescent indicators for neuronal activities.•Discussion on new engineering approaches for indicator development.•Identification of key priority areas for future indicator improvement by tool developers.•Practical guidelines for neuroscientists to evaluate and choose the most appropriate indicators. Genetically encoded fluorescent indicators have transformed the way neuroscientists record neuronal activities and interrogate the nervous system in vivo. In this review, we discuss recent advances and new additions to the toolkit of indicators for calcium ion entry, membrane voltage change, neurotransmitter release, and other neuronal molecular processes. We highlight new engineering approaches for indicator design and development, and identify key areas for future improvement. From molecular tool developers’ perspective, we aim to provide practical information for neuroscientists to evaluate and choose the most appropriate indicators for enabling new insights into brain function.